Paper is made by forming a mat of fibers, normally wood fibers, on a moving wire screen. The fibers are in a dilution with water constituting more than ninety-nine percent of the mix. As the paper web leaves the forming screen, it may be still over eighty percent water. The paper web travels from the forming or wet end of the papermaking machine and enters a pressing section where, with the web supported on a felt, the moisture content of the paper is reduced by pressing the web to a fiber content of between thirty-five and fifty-five percent. After the pressing section, the paper web is dried on a large number of steam heated dryer rolls, so the moisture content of the paper is reduced to about five percent.
The dryer section makes up a considerable part of the length of a papermaking machine. The web as it travels from the forming end to the take-up roll may extend a quarter of a mile in length. A major fraction of this length is taken up in the dryer section. As the paper industry has moved to higher web speeds, upwards of four- to five-thousand feet per minute, the dryer section has had to become proportionately longer because less drying is accomplished at each dryer as the paper moves more quickly through the dryers.
One type of dryer, known as a two-tier dryer, has two rows of steam heated dryer rolls four to seven feet in diameter. The dryer rolls in the upper and lower rows are staggered. The paper web runs in a meandering fashion from an upper dryer roll to a lower dryer roll and then on to an upper roll over as many rolls as is required. An upper felt backs the web as it travels over the upper dryer rolls, and leaves the paper web as it travels to the lower rolls. The upper felt is turned by felt reversing rolls spaced between the upper rolls. On the lower dryer rolls the web is supported by a lower felt, which is also turned between lower dryer rolls by lower felt reversing rolls. This apparatus advantageously dries first one side and then the other of the web, however, the paper web is unsupported for a length as it passes from the upper dryer rolls to the lower dryer rolls, and from the lower rolls to the upper rolls. Unsupported paper webs present a problem as web speed increases. At higher web speeds, the paper interacts with the air and can begin to flutter. This fluttering can wrinkle and crease the paper web, seriously damaging the quality of the paper produced. Further, the fluttering can lead to tears and web failure, with all the cost and downtime associated with paper lost during the rethreading operation.
A first approach to overcoming this problem was to use a single felt or a wire which traveled with the paper web over both the upper and lower dryers so that the paper was supported through the open draws. This approach limited paper flutter in the open draws, but, because the blanket was disposed between the paper web to be dried and the lower dryer rolls, the effectiveness of the lower dryer rolls was substantially diminished.
A further dryer development is the single tier of dryer rolls with vacuum reversing rolls disposed therebetween. The vacuum rolls, such as shown in U.S. Pat. No. 4,882,854 (Wedel, et al.), use vacuum to clamp the edges of the paper to the reversing roll to prevent edge flutter, and use drilled holes or central grooves to allow passage of the trapped boundary layer between the blanket and the reversing rolls.
Single tier dryer systems are successful in increasing the drying rate and shortening the dryer section of a papermaking machine. It is necessary in order to dry both sides of the web effectively to employ both top felted and bottom felted single tiers of dryers. Bottom felted dryers have the disadvantage in that removing broke from between the felt and the dryer can be a difficult and time consuming operation. On the other hand, in the top felted dryers, when the felts are loosened, broke drops with relative ease out from between the felt and the dryer rolls. A further possible problem with single tier dryers is the sequential drying of first one side and then the other. When both sides of the sheet are not dried simultaneously curl can develop in the paper due to the effect of drying on the dimensions of the fibers on one side of the sheet as opposed to the still wet fibers on the other which can produce a tendency for the paper web to curl both in the cross machine and in the machine direction.
What is needed is a shorter dryer section which dries both sides of the web simultaneously and which facilitates rapid clearing of broke from the dryer section.